In electrostatic copying, an electrostatic latent image is formed on an element. That image can be developed into a visible image by the application of toner powder thereover. The resulting toned image is then transferred from the element to a receiver to which the transferred toned image is fixed usually by heat fusion. The transfer of the toned image to the receiver has usually been electrostatically accomplished using an electrostatic bias applied between the receiver and the element.
In order to produce copies of very high resolution, it is necessary to use toner particles that have a very small particle size, that is, less than about 8 microns.
Electrostatic transfer of very small toner particles, particularly of those having a particle diameter less than about 12 microns, is difficult to accomplish because, during such transfer, the forces holding the particles to the element are greater than the electrostatically generated transfer forces. To avoid this problem, a non-electrostatic transfer process must be used with toned images of such particles.
One suitable transfer process is provided by a thermally assisted transfer procedure. A receiver is heated prior to entering a transfer nip so that, in the nip, the surface temperature of the receiver is, typically in the range of about 60.degree. to about 90.degree. C. Upon entering the nip, the receiver is contacted against the toned image formed on the element. The heated receiver sinters the toner particles, causing them to stick to each other and to the receiver, thereby effecting a transfer of the toned image from the element to the receiver. The element and the receiver are separated, and then the transferred toned image is heat fused or otherwise fixed to the receiver. This process is useful, but suffers from the disadvantage that, the receiver must be smooth. Moreover, it is frequently necessary to use a low surface energy element or coat said element with a release aid to effect said transfer.
Another suitable process is provided by a modified thermally assisted transfer process. Here, a receiver is provided with a thermoplastic polymer coating which may have a layer of a release agent thereon. The coating polymer T.sub.g is not more than about 10.degree. C. above the toner polymer T.sub.g. A toned image is transferred using a procedure similar to that employed in the above described thermally assisted transfer process. Toner particles of the image adhere to or become partially embedded in the polymer coating. Subsequent to transfer the image is fixed. Scattering is avoided and substantially all toner powder is transferred. This process suffers from the disadvantage that specially prepared receivers must be used.
However, so far as now known, no thermally assisted transfer process is known by which a high resolution toner powder image comprised of very small toner particles can be transferred from an element to a rough paper, cloth or similar surface without significant loss of image degradation.